Provided in the present invention are a transcription factor composition comprising transcription factors ELL, AFF and CRTC. A mammalian protein expression system may be obtained by co-transfecting an expression vector comprising a gene encoding the three transcription factors and a target gene expression vector into a cell line. The presence of transcription factors ELL, AFF and CRTC in the expression system promotes the transcription and expression of the target gene and may be applied to the production of mammalian protein.